title: Accuracy of conventional oximetry for flow estimation in patients with superior cavopulmonary connection: a comparison with phase-contrast cardiac MRI.
abstract: BACKGROUND
  Cardiac catheterization is routinely used as a diagnostic tool in single ventricle patients with superior cavopulmonary connection. This physiology presents inherent challenges in applying the Fick principle to estimate flow. We sought to quantitatively define the error in oximetry-derived flow parameters using phase-contrast cardiac MRI (CMR) as a reference.
  
  
  RESULTS
  Thirty patients with superior cavopulmonary connection who underwent combined CMR and catheterization between July 2008 and June 2012 were retrospectively analyzed. Estimates of flow and resistance calculated using the Fick equation were compared with CMR measurements. Oximetry underestimated CMR-measured pulmonary blood flow (Qp) by an average of 1.1 L/min per m(2) or 32% of the CMR value (P<0.0001). Oximetry overestimated systemic blood flow (Qs) by an average of 0.5 L/min per m(2) or 15% of the CMR value (P=0.009). There was no correlation between the Qp:Qs ratio derived by Fick and that measured by CMR (ρ(c)=0.01). The error in the Fick Qp correlated moderately with the measured systemic-to-pulmonary arterial collateral flow (r=0.39). The median total oxygen consumption calculated using combined CMR and oximetry data was 173 mL/min per m(2), higher than the assumed values used to calculate flows by the Fick equation. The upper body circulation received on average 51% of systemic blood flow while conducting only 39% of total body metabolism.
  
  
  CONCLUSIONS
  Fick-derived estimates of flow are inherently unreliable in patients with superior cavopulmonary connections. Integrating flows measured by CMR and pressures measured by catheter will provide the best characterization of superior cavopulmonary connection physiology.
authors.affiliation: Division of Cardiology and Department of Radiology, The Children's Hospital of Philadelphia, PA; and Center for Clinical Epidemiology and Biostatistics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia.
authors.forename: Tacy E
authors.initials: TE
authors.lastname: Downing
authors.forename: Kevin K
authors.initials: KK
authors.lastname: Whitehead
authors.forename: Yoav
authors.initials: Y
authors.lastname: Dori
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Gillespie
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Harris
authors.forename: Mark A
authors.initials: MA
authors.lastname: Fogel
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Rome
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Glatz
mesh_terms: D006328:Cardiac Catheterization / Q000379:methods*; D002648:Child; D002675:Child, Preschool; D005260:Female; D005500:Follow-Up Studies; D018729:Fontan Procedure / Q000379:methods*; D006330:Heart Defects, Congenital / Q000175:diagnosis / Q000503:physiopathology* / Q000601:surgery; D006801:Humans; D007223:Infant; D019028:Magnetic Resonance Imaging, Cine / Q000379:methods*; D008297:Male; D010092:Oximetry / Q000379:methods*; D012039:Regional Blood Flow; D015203:Reproducibility of Results; D012189:Retrospective Studies
keywords: bidirectional cavopulmonary shunt; catheterization; collateral circulation; congenital heart defects; magnetic resonance imaging